This application claims the priority of German application 100 22 916.6, filed on May 11, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a cross member as part of a motor-vehicle floor assembly which, on both sides, has a connecting region which comprises an attachment region for the attachment of a rear axle unit and a spring-receiving region which is adjacent to the attachment region and is intended for the fixing in place of a helical spring. The cross member is provided with a stiffening structure in the connecting region.
A cross member of this type is known, for example, in the series vehicle construction of the C-class of Mercedes-Benz. The cross member has, on both sides, connecting regions for the attachment of the rear axle, and for the fixing in place of the rear axle spring struts. The connecting regions therefor firstly have attachment regions to which the rear axle unit is screwed in its rear coupling point, and secondly receiving shells to which are attached spring plates for receiving helical springs which are used to support the rear axle unit with respect to the cross member. During operation of the vehicle, rear axle forces and spring forces are introduced into the connecting regions of the cross member, which results in high bending loads in these regions. In order to obtain a local increase in the stiffening and strength, the connecting regions are reinforced by a plurality of additional metal plates which span the base and the upper edge of the cross member in the connecting region and are welded to the latter via MAG welding seams. These additional metal plates, which are connected linearly to the cross member, therefore form reinforcing ribs which result in a specific increase in stiffness of the connecting regions.
However, in terms of manufacturing engineering, the linear welding of the reinforcing metal plate to the cross member signifies a considerable additional outlay on the production of the floor assembly: this is because, whereas otherwise only spot welding is used for connecting the individual metal plates to the floor assembly, for the attachment of the reinforcing metal plates, a seam-welding process is required which is associated with the use of additional welding apparatuses (for example, MAG welding robots) and in consequence incurs considerable additional costs. Furthermore, in the case of metal plates having a heat-treated surface, the MAG welding can be used only to a limited extent, sincexe2x80x94depending on the type of surface coatingxe2x80x94the MAG welding can result in the heat-treated layer being burnt away and therefore in a substantially increased susceptibility of the metal plates to corrosion.
An object of the invention is therefore based on developing the known cross member in such a manner that a specific, local increase in the stiffness and strength of the connecting regions is obtained and, at the same time, the disadvantages associated with seam welding are avoided. Furthermore, the connecting regions are to be designed in such a manner that a highly precise alignment of the rear axle unit with respect to reference points on the cross member is made possible.
According to the invention, this object is achieved by the stiffening structure being of box-shaped design and spanning both the attachment region and the spring-receiving region and in that the stiffening structure is connected to the cross member by means of spot welding.
In certain preferred embodiments of the invention, the connecting region is reinforced with a box-shaped stiffening structure which spans both the attachment region and the spring-receiving region and which is connected to the cross member with the aid of a spot-welding process. Because of its box shape, the stiffening structurexe2x80x94although it is only connected at points to the cross memberxe2x80x94brings about an increase in the strength of the connecting region. The increase is many times higher than the conventional increase in strength obtained by seam-welded additional metal plates. Therefore, the refinement according to such embodiments of the invention of the connecting region firstly obtains a significant increase in the strength of this heavily loaded zone and secondly, the putting in place of the stiffening structure is substantially less complicated in terms of manufacturing engineering and therefore more cost-effective to bring about. The stiffening structure can be fitted in the cross member with the aid of the point welding used throughout in the vehicle body shell, and so, an additional welding process is not necessary for this type of stiffening. As a result, the outlay on apparatus for joining together the floor assembly is considerably reduced.
According to certain preferred embodiments of the invention, the reinforcing structure expediently comprises a basic element and a stiffening element, the basic element overlapping in the attachment region in a planar manner over the base of the cross member and having two side walls which protrude approximately perpendicularly with respect to the attachment region, while the stiffening element is of U-shaped design and overlaps in a planar manner both over the side walls of the basic element and over the side walls of the cross member. The basic element is used for the specific reinforcing of the attachment region and is connected thereto by welding points. The stiffening element forms a clasp-like reinforcement of the cross member to whose side walls it is connected, and also, because of its planar connection to the side walls of the basic element, brings about a stiffening connection between the basic element and the side walls of the cross member. The stiffening structure therefore comprises just two individual parts which are placed into the cross member without an additional outlay and are connected thereto by spot welding. Therefore it constitutes a very simple and good-value alternative to conventional stiffening means.
According to certain preferred embodiments of the invention, the welding points which connect the reinforcing element to the basic element and the side walls of the cross member are situated on surfaces which lie approximately perpendicularly with respect to the base of the cross member (and therefore with respect to the attachment region and the spring-receiving region); therefore, when the cross member is subjected to bending and compression loading, primarily shearing forces and only very slight tensile forces act on these welding points, which ensures that these welding connections have great durability and load-bearing capacityxe2x80x94even when subjected to large bending moments.
Furthermore, one of the side arms of the U-shaped stiffening element is expediently designed, in certain preferred embodiments, as a projecting lever in the form of a metal plate which overlaps both over the attachment region and over the spring-receiving region and is arranged approximately perpendicularly with respect to the spring-receiving region. This lever is connected in the attachment region to a side wall of the basic element and in the spring-receiving region to the cross member by welding points; the lever therefore couples together the two regions, in which compression and bending moments are introduced into the cross member, in such a manner that a favorable combining of the different compression and bending forces is achieved. The stiffening structure therefor achieves integration and partial compensation of the spring and axle forces. In order to ensure that the attachment region is coupled to the spring-receiving region in as stable a manner as possible, it is recommended to provide the lever with a stiffening bead which spans both the attachment region and the spring-receiving region.
According to certain preferred embodiments of the invention, the stiffening element is a sheet-metal bending part, since a part of this type can be manufactured at good value and in a simple manner. In order to increase the strength of the stiffening structure, it is advantageous to provide the stiffening element with notched angular stiffening areas in the region of the bending edges.
According to certain preferred embodiments of the invention, the basic element is expediently an extruded part with a thread formed integrally on it for receiving the rear axle unit. The threaded connecting branch is designed such that it interacts with an adapter on the rear axle in such a manner that there is an optimum introduction of force from the rear axle to the cross member with the welding points, which connect the basic element to the cross member, being subjected only to a minimal tensile load.
In order to ensure that the cross member is effectively resistant to corrosion, it is advantageous, according to certain preferred embodiments, to manufacture the cross member and the stiffening structure from zinc-coated steel plate. Since the individual parts are connected by way of spot welding, the coating is preserved during the joining process, with the result that no subsequent or additional corrosion protection has to be provided.
According to certain preferred embodiments of the invention, a stiffening structure comprising a basic element and a stiffening element nested into the basic element furthermore enables the rear axle (which is to be screwed onto the basic element) to be aligned in a highly precise manner with regard to reference points on the cross member.
According to certain preferred embodiments, the basic element and the stiffening element are first of all placed together into the cross member. The basic element is then displaced with respect to the cross member in such a manner that the rear axle screwing-on point on the basic element comes to lie at a predetermined position with respect to a fixing point on the cross member; in this position, the basic element is connected to the cross member by welding points. The stiffening element is then displaced with respect to the basic element in such a manner that its central part comes to lie flat against a side wall of the cross member, and in this position is connected to the basic element and the cross member by welding points. This installation method firstly permits a flexible, highly precise alignment of the basic element (and of the screw-on point of the rear axle unit) with respect to the cross member; and secondly, it is ensured by way of the subsequent alignment of the stiffening element with respect to the basic element and the cross member thatxe2x80x94irrespective of dimensional inaccuracies of the cross memberxe2x80x94good stiffening of the connecting region is ensured.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.